Effects of manure application and plowing on transport of colloids and phosphorus to tile drains

Preferential flow and particle-facilitated transport may be largely responsible for observed leaching patterns of strongly sorbing contaminants such as phosphorus. A series of field experiments was performed to investigate the effects of slurry application and plowing on the subsurface transport of colloids and P. Two 25-m(2) plots at a structured sandy loam site were irrigated on six occasions during 1 yr. Effluent sampled in tile drains below the plots was analyzed for turbidity and fractions of dissolved (<0.24 mu m) and particulate inorganic and organic P. The observed flow conditions indicated macropore flow. The particle concentration in the effluent was initially high, peaking before flow peak, and later gradually decreased with flow rate. The colloid leaching pattern was attributed to an initial depletion of high colloid concentrations in the pore water and an eventual diffusion-limited release of colloids from immobile intra-aggregate water to mobile water. Seasonal variability and management practices caused significant variations in the leaching of P forms. After slurry application dissolved P dominated P loss to the drains. At the events in autumn and winter, particle-facilitated transport of P came to dominate over dissolved P transport, reaching a maximum of 80% of P loss. Results suggested that plowing increases the risk of particle-facilitated and dissolved P leaching in rainstorms shortly after the inversion of the soil. We observed an almost fourfold increase in the leaching of P per volume of leachate when comparing irrigation experiments before and after plowing. Amounts of P associated with particulate matter in drain water were constant within events, but varying between storms. For Danish structured clay soils, P density in leached particles was found to range between a maximum of 6 mg P g(-1) for soils having recently been fertilized and an approximate minimum of 3 mg P g(-1) for soils not recently fertilized.